1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same. More particularly, the present invention relates to a zoom lens useful in a broadcast television camera, a video camera, a digital still camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, it has become desirable for photographic optical systems (zoom lens) used in an image pickup apparatus, such as a television camera, a silver-halide film camera, a digital camera, or a video camera, to include a zoom lens with a high F-number value, a high zoom ratio, and a high optical performance. In order to achieve a high F-number value and a high zoom ratio, a conventional zoom lens system includes a four-unit zoom lens including, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power.
U.S. Pat. No. 6,940,656, Japanese Patent Application Laid-Open No. 2004-264459, and U.S. Pat. No. 7,505,213 discuss a four-unit zoom lens, as described above, having a high zoom ratio of about 96 or 50. In addition, U.S. Pat. Nos. 6,404,561 and 6,594,087 discuss a four-unit zoom lens with chromatic aberration corrected by using a lens made of a material having anomalous dispersion.
However, if the zoom ratio becomes as high as about 100, a large amount of variation of various aberrations may occur. Accordingly, in this case, it becomes difficult to achieve a high optical performance over the entire zoom range.
In particular, a large amount of chromatic aberration of magnification (lateral chromatic aberration) and axial chromatic aberration (longitudinal chromatic aberration) may occur at the zoom position close to the telephoto end. Therefore, it is important to effectively correct aberrations in the secondary spectrum as well as those in the primary spectrum to form a high quality image.
In a positive-lead type four-unit zoom lens, in order to achieve a high zoom ratio of about 100, to effectively correct chromatic aberration, and to achieve a high optical performance, it is important to appropriately set the configuration of lenses of a second lens unit, which primarily performs variable magnification.
If the lens configuration of the second lens unit is not appropriately set, a large amount of variation of aberrations, such as chromatic aberration, spherical aberration, flare, coma, and color difference of spherical aberration, may occur during zooming. It therefore becomes difficult to achieve both a zoom lens having a high zoom ratio and a high optical performance at the same time.
In particular, axial chromatic aberration may increase at the telephoto end. In addition, residual aberration in the secondary spectrum may occur in too large an amount to effectively correct.